[tab_under] Log RAPPOR for the opener URL

We want to log UKM for this data in the near future, but that might not
be ready for some time. In the meantime we might be able to get decent
insight from RAPPOR.

Bug: 661629
Change-Id: Ibabdb5d76cf59d31d866c9177e74dcd20cfd964d
Reviewed-on: https://chromium-review.googlesource.com/735781
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511272}
3 files changed